In order to reduce the time necessary to identify the best fitting surface and to ensure that the method is applied by all users in the same way the mathematical extrapolation procedure has been implemented into a Microsoft Excel based computer program named DHWScale.

For the solar domestic hot water system(s) of the product line that have/has been tested with the DST — method the following inputs have to be entered in the Excel sheet:

• For each tested system of the product line

— Collector area

— Storage tank volume

— Solar fraction fsol

• Number of systems tested from the product line

• Location (only Athens available up to now)

• Daily hot water consumption

With these data the program automatically computes the best fitting surface for the specific location and hot water consumption. When the corresponding surface is known the solar fraction can be computed for arbitrary sizes of collector area and storage tank volume.

It is possible within the Solar Keymark scheme rules to test and certify thermal solar collectors as families. This reduces the effort for the testing by far.

A similar procedure for SDHW systems is now in the stage of development among testing institutes in Europe. The “Centre Scientifique et Technique du Batiment” (CSTB) in France

developed the “Solen software” for the calculation of the efficiency of solar thermal heating systems in buildings according to prEN 15316-4.3:2006 [3].

The software has now been used to extrapolate between two tested forced-circulation systems. The systems were tested at Fraunhofer ISE using the Dynamic System Testing (DST) procedure according to EN 12976-2. The basic differences between the two systems are the size of the collector array and the size of the storage tank

System A: 2 collectors, Aa=4,72 m2; Storage tank, 295 l

System B: 3 collectors Aa=7,08 m2; Storage tank, 380 l

The following table shows the deviation between test results and simulations done using the “Solen software”. The simulations are adjusted with the test result of the other system in the “system family”. It is seen that for the location Davos, for instance, the deviation between test result and the simulation is smaller than for Athens. Looking at this particular system family the deviation for the locations Stockholm and Davos are very small. More tests and simulations have to be done to validate the procedure.

H. Drtick*, H. Mtiller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, 70550 Stuttgart, Germany Tel.: +49 711 / 685-63553, Fax: +49 711 / 685-63503 Corresponding Author, email: drueck@itw. uni-stuttgart. de

Abstract

In the years 2000 and 2001 the first edition of the European standards for solar thermal systems and components was issued and started to replace all related national standards /1/. The three standard series are related to solar collectors as well as to factory made and custom built solar thermal systems. During the past four years the standards were revised and updated.

Based on the European standards, Solar Keymark certification was established in 2003. The Solar Keymark is the official CEN certification scheme for thermal solar collectors and factory made thermal solar systems /2/. Although the Solar Keymark is still relatively young, more than two thirds of all solar thermal collectors sold in Europe are already qualified with a Solar Keymark label. The specific Solar Keymark scheme rules forming the basis for Solar Keymark certification were revised and updated during the past two years. This was done to adopt the Solar Keymark certification process to present developments and to make Solar Keymark certification of factory made systems less expensive by introducing a so-called “flexible Solar Keymark certification” for system families or product lines respectively.

This paper describes the important changes and highlights resulting from the revision of the European solar standards. With regard to Solar Keymark certification, notable changes in the specific Solar Keymark scheme rules will be pointed out and the approach for flexible Solar Keymark certification will be discussed.

Keywords: European standards, Solar Keymark, testing, certification

1. Introduction

The solar thermal market is growing very dynamically. In order to ensure a certain amount of transparency and quality as a basis for a sustainable market development the existence of uniform standardised test procedures and product certification schemes are very important aspects.

With regard to the elaboration of European standards for solar thermal products the work started almost 15 years ago with the establishment of the European Standardisation Committee CEN TC 312 (CEN: Comite Europeen de Normalisation; TC: Technical Committee) in the year 1994. This activity was based on a proposal of the European manufacturer association ESIF (European Solar Industry Federation) which is today named ESTIF (European Solar Thermal Industry Federation). In the standardisation committee CEN TC 312 experts from industry as well as from research and testing institutions work, divided into several working groups, on aspect related to standardisation.

/

This first set of European standards for solar collectors, ‘factory made systems’ and ‘custom built systems’ was issued in 2000 and 2001 /1/. During the last two yeas these standard were revised and updated. The most important aspects resulting from this activity are described in chapter 3 to 5.

The Solar Keymark is the official CEN certification scheme for thermal solar collectors and factory made thermal solar systems. It requires that the products fulfil the requirements of the European Standard series EN 12975 and EN 12976 and that this is confirmed by an accredited testing laboratory. Furthermore, additional requirements such as yearly inspection of the production line and physical inspection of the product itself every second year, have to be fulfilled.

Although the Solar Keymark is relatively young, as it was introduced to the market in 2003, up to now (summer 2008) approximately two thirds of all solar thermal collectors sold in Europe are already qualified with a Solar Keymark certificate. An overview on Solar Keymark certification is given in chapter 6.

4.
Conclusion

Side-by-side tests of seven differently designed evacuated tubular collectors were carried out in an outdoor test facility. The observations from the measurements show that the direct flow ETC and the all-glass ETC have relatively high thermal performance m2 transparent area. The all-glass ETC with solar collector fluid in the tubes and the double-glass ETC with heat pipe perform relatively better in summer than in the rest of the year. This behaviour is insignificantly influenced by the mean collector fluid temperature. The heat pipe ETC with flat fin performs better than the ETC with curved fin in most of the test period and the superiority will increase in winter periods and in periods with high mean solar collector fluid temperature.

References

[1] Z. Q. Yin, “Development of Evacuated Tubular Collectors in China”, Proceedings of the Solar Thermal Industry Forum, Munich, April 22, 2008.

[2] W. B. Koldehoff, “The Solar Thermal Market-Today and Tomorrow”. Proceedings of the Solar Thermal Industry Forum, Munich, April 22, 2008.

[3] Z. He, H. Ge, F. Jiang, W. Li. A Comparison of Optical Performance between Evacuated Collector Tubes with Flat and Semicylindrical Absorbers. Solar Energy, 60 (2), 1997, PP. 109-117.

[4] J. Fan, J. Dragsted, S. Furbo. Side-by-side Tests of Differently Designed Evacuated Tubular Collectors. Proceedings of the 2007 Solar World Congress, pp. 634-637, Beijing, China, 2007.

After a market analysis, which included all major distributors, a selection of 58 collector glazing types were chosen in the beginning of this long-term investigation (1984). An overview of tested samples is given in Table 1. These glazing types cover a variety of different material and plate types. Although the selection was made in 1984, the results still provide important information regarding the materials currently available on the market.

Two exposition sites with different climatic conditions were chosen (see Table 2.); Rapperswil representing a sub-urban location is home of the SPF institute. The alpine site of Davos is characterized by higher irradiation and lower temperature and air pollution.

Table 2. Main climatic parameters of the exposition sites.

Altitude

Total annual insolation Annual UVA insolation Annual UVB insolation Yearly mean temperature Site character Air pollution sources
417 AMSL

1093 kWh/m2 per year 60.7 kWh/m2 per year 2.08 kWh/m2 per year

9.3 °C Suburban

Train station and industries

Five samples of each glazing type were exposed at the two sites. Each sample covered a “mini collector” [1] which consists of a non-insulated box of solar selective coated stainless steel facing south at an inclination of 60°. One sample from each type was collected, analyzed and stored following 40 days, 1, 3, 10 and 20 years of exposure.

The challenge of the estimation of the real existing loss potential at solar energy systems caused by severe hailstorms is given in the combination of high accounts on, at present, relative small areas, aggravated also by the high spatial concentration of such thunderstorms. This is also the reason why insurance and re-insurance companies accept such losses tacitly up to now and don’t itemize damages at solar energy systems in their loss statistics separately. Nevertheless, it is quite clear that the risk of damages on solar energy systems will enormously increase if we consider the rapid development of the solar thermal as soon as the PV-market in Europe in the last decades and if we also consider the aspiration of the EU to enhance the percentage of sustainable energy up to 20 % until 2020 and up to 50% until 2050 related to the overall energy demand, Fig. 4 and Fig. 5 show the annual rates of growth of solar thermal collectors and PV-modules which are registered up to now as well as predicted until 2020.

Also the establishment of solar thermal as well as photovoltaic power stations for the industrial electricity generation and the increasing installation of large solar thermal systems to supply local heat grids or solar driven cooling systems as well as the furnishing of process heat for industrial processes results in a higher potential of economical losses. 88.8% of the present installed collector area of solar thermal systems in Germany are small systems up to 20 m2 Systems larger than 20 m2 are only 11.2%. Systems over 50 m2 even only 1.7%. For the compliance of the achieved objectives of the EU, the amount of large solar systems has to be increased appreciably. The 2007 published sustainability study of the Sarasin Bank predicted the annual growth rates in the field of large solar thermal systems as given in Fig. 6.

A further aspect which will influence the increase of the economical loss potential is given by the architectural integration of solar thermal collectors and PV-modules into the building shell. Solar energy systems will no longer be installed as several patchworks at the existing building shell but more and more as an integrated component of the building shell. Apart from the function just as an energy collecting device such integrated components have to fulfil other additional functions. Moreover, the efforts to exchange such integrated components in the case of some damages will be more expensive.

The optical efficiency based on a surface reflectance measurements is 0.94, the gap between reflective surface and the absorber fin is 4 mm and the absorptance is 0.95. The first gap loss (green rays) is detected at an incident angle of 44 degrees which is depicted as a decrease in the optical efficiency seen in Fig. 6.

In Fig. 6 gap losses separate into roughly two ranges. A flat response occurs between 80 and 100 degrees and an abrupt efficiency drop occurs at 44 degrees. To show that the gap loss is the cause

of the optical efficiency drops, another simulation is run in which there is no gap loss. See Fig. 7. The graph depicts a rounded distribution with no abrupt jump in efficiency at any nominal angle.

To understand the nature of gap loss, the gap loss is plotted over the incident angles in Fig. 8. The gap losses are separated into roughly two levels.

The reflectivity is now changed to 0.7 to achieve an optical efficiency curve that has a shape that is more of a dished appearance around the 90 degree incident angle. See Fig. 9.

2.1. The education and training for project design professionals

Portuguese Civil Engineers are the legal responsible professionals for elaborate the water supply design projects. Until now they had to deal with simple equipments of hot water production but now they need to calculate a more complex and integrated system. After consulting a sufficient number of senior project design Civil Engineers they have responded that they are not very comfortable with this new technology and with all that is related with mechanical equipments. Traditional projects in the pass never forced them to know more about equipment subjects. Buildings are until now predominantly constructive but they are aware that intelligent and automatic buildings are a future inevitable reality. Despite there are some standard systems that function like a kit module, it is not sufficient to respond efficiently to the building design demand. In some cases, they are letting the design of solar collector system to be developed later by the installation firms with negative consequences on final building quality. Thus, it is absolutely essential that official institutions promote and stimulate even more training for these professionals. The process is been slow but some are making efforts to get training. Also, recent graduate Civil Engineers, who have just finishing theirs courses, faced almost the same problem mainly because many of the Institutions of Higher Education in Portugal still do not provide the required competencies on these subjects. In most of the courses there is still not an adequate and integrated group of curricular subjects that goes deep in these matters, providing the minimal competencies to accomplish sufficient professional practice in this area. Therefore, it would be necessary to make a great effort to introduce on the curricular course structures even more contents on those matters. And we can not forget other important aspect, as the firm’s know-how is still not very high there is always a tendency to charge more for the installation and maintenance operations and give not so qualified engineering consultant. Now, these firms must be prepared to correspond to a more informed attitude from design professionals.

The Input-Output Controller is a simulation based failure detection method available on the market since 2007. The first variant of the method monitors only the energy yields in the solar circuit. Furthermore, two temperature values in the storage are used as input for the simulation. A second approach also includes the buffer storage discharging. The IOC compares the daily measured and expected energy yields in the solar loop. The standard uncertainty (o) of the IOC-procedure, including measurements and yield calculation, is about 7 % (o). If the difference between measured and simulated yield is larger than 20 % (3 o) a fault is detected. This leads to a 99 % reliability for a correct fault prediction. Below a yield of 1.5 kWh/m2d the uncertainty margins are higher. There is a failure tree to establish if the fault occurred inside or outside the solar loop, and if it is for example the control or the solar station which causes the problems. The IOC is sold for 1190 € inclusive temperature and irradiance sensors, but without volume flow measurements. To be able to check the performance from home an extra data logger is necessary [9; 10].

3.3. Kassel University method (KU)

At Kassel University a failure detection method was developed, that combines a static algorithm based function control with dynamic simulation based failure detection [11]. The method consists of three steps. In the first step it is checked if too much data is missing due to data gaps and sensor defects. A minimum of 95 % of data points should be available to continue with the failure detection. In a second step a plausibility check is carried out, in which the correct operating of individual components is checked, similar to the approach used in [3]. The third step is a simulation based step in which the system is modelled with TRNSYS. Measured and simulated energy gains are compared at the heat exchanger for charging and or discharging the storage unit. If the difference is larger than the uncertainty margins on both sides an error is reported [12; 11].

Several failures were detected and partially identified. These were for example air in the collector field and a calcified heat exchanger. This approach is being further developed.

3.4. Guaranteed Solar Results (GRS)

In Guaranteed Result of Solar Thermal Systems the energy yield is guaranteed by the seller/builder of the system. Sophisticated measurement equipment is installed and monitors the system, costs for the measurement equipment and one year of operation are in the range of 10 k€. Daily averaged and monthly measured values are sent. Measured yearly energy yields are compared to simulations with f — chart, a simple simulation program, although also other simulation programs could be used. A comparison on a shorter basis is not possible, due to limitation of the simulation program. Large failures on a yearly basis can be detected; however failure analysis is not possible [13; 14].

A complete research and testing laboratory for scientific research in the field of solar energy utilization was built near Tripoli, Libya, for the CSES of the National Office for Research and Development by the general contractor, Bavaria Engineering GmbH, together with several project partners. TUV Rheinland Immissionsschutz und Energiesysteme GmbH, which recommended PSE AG as provider of the indoor/outdoor test stand for solar thermal collector testing, was project partner especially for the solar thermal test systems.

This paper describes the concept and realization of the test stand for solar thermal collectors. PSE AG and Fraunhofer ISE were able to use their years of experience with indoor and outdoor test stands and to develop the technology further.

PSE AG’s next project consists of the installation of a complete indoor test stand for solar thermal collectors at the French national institute for solar energy, INES, in the fall of 2008.

Numerous innovations will also be realized in this test stand. Besides motor-driven lamp positioning, all test-stand-specific parameters will be centrally controlled and recorded so that tests can be easily documented and reproduced.

We are glad to be able to use our expertise for the realization of test stands for solar thermal collectors.

References

[1] Zahler, C.; Luginsland, F.; Haberle, A; Rommel, M.; Koschikowski, J.;

Fertigung und Installation eines Sonnensimulators fur das GREEN Labor an der Pontificia Universidade Catolica de Minas Gerais in Brasilien,

OTTI: 15. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2005 pp 462-467

[2] Haberle, A.; Berger, M.; Luginsland, F.; Zahler, C.; Rommel, M.; Baitsch, M.;Henning, H.-M.; Linear konzentrierender Fresnel-Kollektor fur Prozesswarmeanwendungen,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 185-190

[3] Haberle, A.; Luginsland, F.; Zahler, C.; Topor, A.; Reetz, C.; Apian-Bennewitz, P.;

Ein praziser undpreiswerter Sonnenstandssensor,

OTTI: 16. Symposium Thermische Solarenergie, Bad Staffelstein, Germany 2006 pp 144-149